Modern automobile side and rear lights, or windows, are being produced more and more with varying degrees of curvature, as opposed to flat, planular panes.
Normally, such window pieces are manufactured or blanked, and tempered, following essentially the same procedures employed for the production of plate glass.
If a curvature is desired in a given blank, it is simply heated to a temperature in the vicinity of 688.degree. C., at which point it is ready to be subjected to bending or curving stress employing any number of suitable molding or pressing techniques, which actually take place slightly lower than furnace ambient, in the vicinity of 677.degree. C., for example. The glass, during the bending operation, still has sufficient surface hardness such that the molding or bending member does not disturb the glass surface with which it comes into contact.
One of the preferred methods for bending and tempering automotive side and rear lights (hereinafter sometimes referred to as "quarterlites") is that described in some detail in U.S. Pat. No. 4,282,026, "APPARATUS FOR BENDING AND TEMPERING GLASS" incorporated herein by reference.
The phase of the patented process, which is of special significance in connection with the instant invention, is that step wherein the preheated quarterlite is vacuum lifted from the roller hearth or conveyor of the initial heating furnace, and transferred to the mold for bending or shaping under the force of gravity, while still hot.
Transfer from the furnace hearth rollers to the bending mold is accomplished by a head which lifts the hot glass for transfer under a vacuum. The vacuum head is faced with a somewhat resilient pad fashioned of ceramic fiber, one type known as "Fiberfrax", manufactured by the Carborundum Company. The relatively soft, and very slightly resilient, padding functions as a cushion to minimize any tendency to disturb the polished surface of the hot quarterlite during transfer to the bending mold.
As is well known, quarterlites are secured in their openings, upon installation, by an adhesive between the glass and an inner flange of the opening, the glass being pushed into adhering contact from the outside. Following fixing of the glass in the foregoing manner, a suitable, decorative (usually metallic) frame is put into place from the outside, to cover and seal the glass edge, and to also function as an exterior decorative frame for the glass.
Of late, automotive manufacturers have found that the appearance of the quarterlite is greatly enhanced, and the underlying layer of adhesive is both masked and protected from UV degradation, by applying a relatively narrow, opaque, colored band or border around the edge of the glass, on the inner surface; this band frames the edge of the inner surface, and may vary anywhere from an inch to six inches wide.
Preferably, the combination decorative and protective colored layer which completely frames the inner surface of the glass, is sufficiently opaque to mask and protect the adhesive from UV degradation. It is also preferably of a relatively dark color, black or dark gray, so as to be compatible with the myriad colored finishes currently applied to the automotive exterior. That is, a narrow portion of the band may be viewed from outside the car if it extends past the decorative molding.
Because of its relative durability, it has been found that the colored band is best applied in the form of a vitreous, glass enamel around the edge of the inner surface of a quarterlite, followed by firing to fusion, as is well known in the glass enamelling art.
However, the need for the added manufacturing steps of screening the glass color onto the window pane to be decorated, drying, followed by a firing operation, cooling, etc., prior to bending or forming, proved to be prohibitively expensive.
The idea as then conceived that, if the glass pane were to receive the screened-on glass enamel band, prior to the bending operation, the glass enamel could then be fired as a consequence of preheating the glass pane at the approximately 688.degree. C. temperature, preparatory to bending at or about 677.degree. C.
However, this procedure had one, very serious, disadvantage. When the Fiberfrax faced vacuum head came into contact with the still-soft, enameled surface of the vitreous, decorative band, two undesirable events occurred. First, the Fiberfrax-covered face of the transfer head left a faint, patterned imprint in the soft glass enamel, to which a few random fibers tended to stick, while at the same time, a sufficient amount of the somewhat tacky glass enamel was abraded, and/or absorbed onto or into the Fiberfrax material which, probably because of the fluxing action of the enamel glass, tended to erode the Fiberfrax face, thereby requiring frequent, and expensive replacement. During replacement, the bending member obviously was out of service, necessitating a shutdown of the bending line, etc.
One solution was to increase the refractory nature of the glass enamel by conventional means, whereby its fusion temperature was sufficiently high so that it did not adhere to the Fiberfrax facing. But then it could not be fired out and matured as a consequence of, and simultaneously with, the preheating operation.
It has been discovered by the instant inventors however, that by employing the highly specialized glass enamel formulation of this invention, a glass enamel can be produced which will provide the required fired and matured glass color band or border around the edge of the auto window pane, at or about bending temperatures, but which at the same time will be totally resistant to abrasion by, or adherence to, the Fiberfrax pad of the vacuum head of the transfer member discussed above.
Simply stated, the glass enamel of this invention has been formulated in such a way that it fuses to maturity at the temperature at which the quarterlite is preheated preparatory to the bending operation. But then, as a consequence of that preheating, it is theorized that the cured glass enamel immediately undergoes a partial devitrification, or recrystallization phenomenon, whereby, at its softening temperature, its surface remains sufficiently hard, such that there is no adhesion between the soft enamel and the Fiberfrax.
Thus, by the time the preheated, preenameled glass pane reaches the bending stage, and comes into contact with the Fiberfrax pad which covers the vacuum head, the glass enamel, though fused and "cured", is, and remains, sufficiently hard such that it completely resists yielding, to any noticeable degree, to the stress of the Fiberfrax-faced transfer member, and as a consequence, none of the glass enamel is abraded onto, or absorbed into, the Fiberfrax mesh to cause the damage described above. Furthermore, the Fiberfrax leaves no imprint in the fired glass enamel surface.